Various embodiments are based on a circuit arrangement for conversion of an input AC voltage to a DC voltage.
Parallel operation of low-voltage halogen lamps and LED retrofit lamps is known from U.S. Pat. No. 6,380,693B1. The problem of efficient current limiting for operation of the light-emitting diodes has, however, not been solved there, with only resistors being provided there, for current limiting. As a result of the parallel operation of light-emitting diodes and low-voltage halogen lamps, the power consumed by the light sources falls below the necessary power consumption which is required, for example, in order to allow an electronic transformer for low-voltage halogen lamps to operate. Normally, electronic transformers for low-voltage halogen lamps have no DC voltage as an output signal, but operate the low-voltage halogen lamps with an AC voltage. This AC voltage is at a high frequency, which normally corresponds to the clock frequency of the electronic transformer, and is amplitude-modulated at the input mains frequency.
EP 1 076 476 A2 discloses an apparatus for operation of at least one light-emitting diode, which can be connected to an electronic transformer for halogen lamps, with the apparatus containing a rectifier and a storage circuit.
The fundamental problem of operating one or more light-emitting diodes on an electronic transformer for low-voltage halogen lamps is to offer a resistive load to the electronic transformer for low-voltage halogen lamps. Many electronic transformers for low-voltage halogen lamps are regulated such that the load current must follow the voltage, since, otherwise, the transformers assume that there is a fault, and switch off. The load current can follow the voltage only by assuming a resistive behavior in the system as a load.
In the case of the apparatus disclosed in EP 1 076 476 A2, a storage capacitor C1 is always charged with both half-cycles of the input AC voltage UET, and is therefore never completely discharged. This in turn leads to a pulsed current being drawn, which does not work in many electronic transformers. Furthermore, in this case, resistors are provided for current limiting, which cause losses and are therefore undesirable.